Technology for beam forming is well known art and can be realized through an array of antennas and phase shifters whereby the phase of each antenna signal is shifted to compensate for the different time of arrival of the signal coming from an arbitrary angle relative to its neighboring antenna element. If radio frequency (RF) phase shifters are used, then the signal of all the antenna elements must be combined to form a single RF signal that is spatially filtered. Such a scheme is not easily tiled or efficient because the array as described can only look in one direction at a time, which precludes true spatial multiplexing. Furthermore, processing RF signals is difficult due to the large insertion loss of most phase shifters, the limited resolution of such phase shifters, and the difficulty of summing RF signals directly in an impedance controlled environment. The element that does the signal summation will be very large in area due to the physically large area of RF summation blocks, such as transmission line based Wilkinson combiners, and it will need a large gain to overcome the lossy signal distribution at RF.
While in theory the number of simultaneous “look” directions of the array can be increased by duplicating the number of phase shifters in each tile, or by using Butler matrices, this approach does not scale well for all of the aforementioned reasons. The RF signal needs to be split into Ns copies, incurring loss, and then each signal path is phase shifted and combined, incurring more loss. A better approach is to use IF or baseband phase rotators, which are usually implemented by taking a weighted combination of in-phase (I) and quadrature-phase (Q) signals. Such a vector combiner is compact, can be realized with all active circuitry, and can provide accurate phase control. In previously described art, building a large array requires performing analog summation of a very large number of output signals (equal to the number of antennas), which is limited by the parasitic capacitance in the output summer structure. Processing multiple streams makes this signal processing and routing even more challenging. In practice, the output of only a limited number of elements could be efficiently processed in the analog domain, limiting the array size to a dozen elements, particularly at microwave and mm-wave frequencies.
Full digital beam forming is very flexible and easily tiled, since each tile element consists of a full radio and digital outputs, but suffers from a very large number of signals that must be digitized (equal to the number of antenna elements) and the vast amount of data that must be transported for centralized signal processing. Recently, researchers at the Berkeley Wireless Research Center (BWRC) have proposed a distributed digital array architecture that eliminates the central processing by performing the beam forming in a distributed fashion. In such an architecture, the number of analog-to-digital convertors (ADCs) is not reduced, but the data transported across the array is cut down significantly.
The challenge is to find an architecture that is amenable to tiling and can also perform beam forming in the analog domain, so that the number of signals to be processed is reduced from the antenna count down to the number of streams (or spatial directions) that are to be processed.